I wonder what would happen if I deleted /opt/amazon/screen_saver, and replaced it with a symlink to something in /mnt/us. /opt isn't FAT right? (I should just check that when I get home).

When I say "what would happen" what I really mean is: would this approach be ignored by the amazon updates? After some investigation it seems that before an official kindle update runs it checks to see that files its about to update haven't changed. I'm betting that it doesn't check directories in the same way, though I've noticed a few odd things about busybox's tar implemementation that would make me not be surprised if a later amazon update simply put it back the way it was.